1. Field of the Invention
The present invention relates to a nitride semiconductor laser device, and more particularly relates to a nitride semiconductor laser device having a ridge waveguide structure.
2. Description of the Related Art
A nitride semiconductor is made of a compound semiconductor including InxAlyGa1-x-yN (0≦x, 0≦y, 0≦x+y≦1), and requirements for a laser device using such a nitride semiconductor is increasing, for use in optical disc systems capable of recording and reproducing large-volume, high-density information such as a next-generation DVD, and for use in electric appliances such as personal computers. Consequently, studies on semiconductor laser devices using a nitride semiconductor have been vigorously conducted. Also, a semiconductor laser device using a nitride semiconductor is expected to emit light over a wide range of wavelengths of visible light, from ultraviolet region to red, and therefore a wide variety of application areas are expected, including laser printers, laser displays, and light source of optical networks.
Particularly, various studies have been conducted on the structure of nitride semiconductor laser devices and structures which allow favorable lateral mode control, on the structures and the like for low power consumption, high output power, high reliability, small size, long operating life and so fourth. Of these, a structure which holds particular promise is one with a ridge waveguide structure. In a nitride semiconductor laser device having a ridge waveguide structure, a ridge is provided to control the lateral confinement of light, and a protective film for electric current constriction and light confinement is formed on the side surfaces of the ridge and the surfaces of the nitride semiconductor layer at both sides of the ridge. Japanese Patent Publication Nos. JP H09-283845A, JP H10-270792A, and JP 2000-12970A disclose such nitride semiconductor laser device, for example.
However, lateral confinement of light in a ridge waveguide type laser device depends on various factors such as the width and depth of the ridge, a material of the protective film etc. Optical confinement is easily affected by the factors other than those to be taken into consideration in the design of the devices, by the factors that may change over time such as adhesion between nitride semiconductor layer and protective film, and controlling the optical confinement has been difficult. That is, even if a protective film having good adhesion with the nitride semiconductor layer is selected, the optical confinement may not be so good. Also, even if a ridge is formed and a protective film is selected in consideration of the refractive index with the nitride semiconductor layer, the protective may not necessarily have good adhesion with the nitride semiconductor layer. Therefore, it is necessary to form a nitride semiconductor laser device with a structure capable of maintaining good performance for long time in terms of both the optical confinement and the adhesion.